The invention relates to a silver catalyst suitable for the oxidation of ethylene with oxygen and a process for the production of said catalyst.
As is known, silver catalysts are used for the production of ethylene oxide by the controlled incomplete oxidation of ethylene with molecular oxygen. Numerous modifications of the silver catalyst have already been proposed. It is known from DE-20 17 733, DE-29 14 640, as well as DE-33 34 347, to precipitate metallic silver from silver-containing suspensions or mixtures of suspensions and solutions on slightly porous catalyst supports. In these catalysts, the silver is present according to experience in a largely continuous layer with variable thickness, which consists of relatively large silver particles and covers the surface of the support as well as the walls of the channels and holes in the support. Such a layer can easily be mechanically damaged; as a result silver dust occurs in the reaction pipe, and the pressure difference between the ends of the reaction pipe increases during the operation. As a result, the gas throughput and the performance of the reactor decrease. In such catalysts, produced according to the "suspension process," the activity and selectivity decrease particularly fast.
It is also known from DE-21 59 346 and EP-0 161 930 to produce silver catalysts according to the impregnation process. In this case, a highly porous catalyst support is immersed in a solution of one or more silver compounds. Such impregnated catalysts do exhibit favorable aging characteristics, but generally at the beginning of their use period, they have a lower activity and selectivity than suspension catalysts. To counteract the decrease in selectivity, the selectivity of impregnated catalysts can be improved according to DE-23 00 512, DE-27 34 912, EP-0 266 015, and EP-0 229 465 by alkali metal and other promoters being additionally precipitated on the support. But according to experience, a favorable doping of promoters for improvement of selectivity results in a loss of activity. By a choice of the type and amount of suitable promoters alone, a highly selective and at the same time highly active catalyst cannot be produced.
According to DE-33 21 895 impregnated catalysts are doped with alkali and alkaline-earth promoters in comparatively high concentration and then heated to temperatures of 450.degree. C. to 800.degree. C. to increase the activity, but according to experience, even with these catalysts, activity and selectivity are still in need of improvement.